The major treatment modalities used by oncologists to treat cancer are surgical resection, radiation therapy, and classical chemotherapeutic drugs. Unfortunately, surgical resection is not a viable option for many tumors or forms of cancers. Further, radiation therapy and chemotherapeutic drugs do not target only diseased cells and therefore, end up damaging healthy cells. Therapeutics that more specifically target tumor cells are being developed by taking advantage of tumor-specific expression of antigens or inappropriate overexpression or activation of specific proteins within tumor cells, but tumor cells are prone to mutation and can become resistant to drugs that specifically target tumor cells.
A new cancer treatment paradigm has emerged that harnesses the patient's own immune system to overcome immunoevasive strategies utilized by many cancers and to enhance anti-tumor immunity. One such strategy is to inhibit negative regulators of immune responses that normally function to maintain peripheral tolerance, allowing tumor antigens to be recognized as non-self entities.
The hematopoietic progenitor kinase 1 (HPK1) is an example of a negative regulator of dendritic cell activation, and T and B cell responses that can be targeted to enhance anti-tumor immunity. HPK1 is expressed predominantly by hematopoietic cells, including early progenitors. In T cells, it is believed that HPK1 negatively regulates T cell activation by reducing the persistence of signaling microclusters by phosphorylating SLP76 at Ser376 (Di Bartolo et al. (2007) JEM 204:681-691) and Gads at Thr254, which leads to the recruitment of 14-3-3 proteins that bind to the phosphorylated SLP76 and Gads, releasing the SLP76-Gads-14-3-3 complex from LAT-containing microclusters (Lasserre et al. (2011) J Cell Biol 195(5):839-853). HPK1 can also become activated in response to prostaglandin E2, which is often secreted by tumors, contributing to the escape of tumor cells from the immune system.